Large harmonics, poor power factor, and high total harmonic distortion (THD) in a utility (power supply) interface are common problems when nonlinear loads such as adjustable speed drives, power supplies, induction heating systems, UPS systems and aircraft converter systems are connected to the electric utility.
In some systems, the interface to the electric utility includes a three phase uncontrolled diode bridge rectifier. Due to the nonlinear nature of some loads, the input line currents can have significant harmonics. For adjustable speed AC motor drive systems with no DC-link smoothing inductor, the discontinuous conduction of the diode bridge rectifier can result in high current THDs which can lead to the malfunction of other sensitive electronic equipment.
One recommended practice to address these issues, IEEE 519, has evolved to maintain utility power quality at acceptable levels. See, "IEEE Recommended Practices and Requirements for Harmonic Control in Electric Power Systems", IEEE PES and Static Converter Committee of IAS, January 1993.
A number of methods have been proposed to overcome the presented problems. See, e.g., J. Schaefer, "Rectifier Circuits: Theory and Design," John Wiley & Sons, Inc., 1965; B. R. Pelly, "Thyristor Phase-Controlled Converters and Cycloconverters, John Wiley & Sons, 1971; R. W. Lye, etc. al, "Power Converter Handbook," Power Delivery Department, Canadian General Electric Company Ltd., 1976; G. Seguier, "Power Electronic Converters AC/DC Conversions," McGraw-Hill, New York, N.Y., 1986; G. Oliver, etc. al, "Novel Transformer Connection to Improve Current Sharing on High Current DC Rectifiers," IEEE IAS Conf., 1993, pp. 986-992; S. Miyairi, etc. al, "New Method for Reducing Harmonics Involved in Input and Output of Rectifier with Interphase Transformer," IEEE Trans. on Industry Applications, Vol. IA-22, No. 5, September/October 1986, pp. 790-797; A. R. Prasad, P. D. Ziogas, S. Manias, "An Active Power Factor Correction Technique for Three-Phase Diode Rectifiers," IEEE Trans. on Power Electronics, Vol. 6, No. 1, January 1991, pp. 83-92; Ned Mohan, "A Novel Approach to Minimize Line-Current Harmonics in Interfacing Renewable Energy Sources with 3-Phase Utility Systems," IEEE APEC Annual Meeting, 1992, pp. 852-858; S. Kim, P. Enjeti, P. Packebush and I. Pitel, "A New Approach to Improve Power Factor and Reduce Harmonics in a Three-Phase Diode Rectifier Type Utility Interface," IEEE Trans. on Industry Applications, Vol. 30, No. 6, November/December 1994, pp. 1557-1564; I. Pitel and S. N. Talukdar, "A Review of the Effects and Suppression of Power Converter Harmonics," IEEE IAS Conference, May 1977; S. Choi, P. Enjeti, I. Pitel, "New Polyphase Transformer Arrangements with Reduced kVA Capacities for Harmonic Current Reduction in Rectifier Type Utility Interface," IEEE PESC Conf, 1995, pp. 353-359; D. A. Paice, "Power Electronic Converter Harmonics: Multipulse Methods for Clean Power," New York: IEEE Press, 1996.
One approach is to use a conventional twelve-pulse diode rectifier which requires two six-pulse diode rectifiers connected via Y-.DELTA. and Y--Y isolation transformers. An interphase reactor is required to ensure the independent operation of the two parallel-connected three-phase diode bridge rectifiers. The operation of the conventional twelve-pulse diode rectifier results in the cancellation of the 5th and 7th harmonics in the input utility line currents.
To increase the pulse number further to 18 or 24, additional diode bridge rectifiers along with complicated multiphase transformer arrangements become necessary, undesirably adding to the cost and complexity of the overall system.